I enter the fever. From my window I see the birth of the seas, hills covered by foam, dead, submerged brides. I am afraid of being found with this vision, that they discover my desire to run after a legion of drowned ones. The body plunges down, it sparkles. I am one with all; my feet liberate me from the way. The sword, the gold of the pond, convulsed. The flame goes up, it cuts the thread of resistance. There is a hand lost for writing, another that rescues it, that supports the needles of being. It does not weave it, it only takes care of the verticality of the dream. No, I don't stop falling. Look at this mauve rain: it has found another lineage, a mystical foretaste, an animal of the depths that remembers itself and remembers us. It is the cold, the exaltation, the volcanic hand that opens you, and pleasure. Do not let go the flower.

You have a great need to be loved, appreciated… Even worshipped. You enjoy luxury, sensuality, and pleasures of the flesh. You look for lovers who know what they are doing. You are not interested in an amateur,unless that amateur wants a tutor. You are fussy and exacting about having your desires satisfied. You are willing to experiment and try new modes of sexual expression. You bore easily and thus require sexual adventure and change.You are more sensual than sexual, but you are sometimes down right lustful.

M for Morgaine (my chosen name)

You are emotional and intense. When involved in a relationship, you throw your entire being into it. Nothing stops you; there are no holds barred. You are all consuming and crave someone who is equally passionate and intense. You believe in total sexual freedom. You are willing to try anything and everything. Your supply of sexual energy is inexhaustible. You also enjoy mothering your mate.

"I was dating Gordon when I met Heather and Jim. Then I started dating Jim too, and Heather started dating Gordon right before he and I broke up," says Noemi. Confused? Tonight I'm having dinner with a group whose unusual lifestyle warrants such introductions. They are a "polyamorous" family – one whose members are openly committed to several lovers at the same time.

Their household, in a quiet neighbourhood on the outskirts of San Francisco, looks like any other. A little boy in pyjamas answers the door when I knock, smiling around a large strawberry stuck in his mouth. His mother Heather, an artist with oval glasses and pink hair, is cooking in the kitchen with her boyfriend Gordon, a computer-network engineer with an understated manner. The dining room is pleasant, airy and smells of roasting chicken. Heather's husband Jim, along with housemates Noemi and Alicia, are bustling about the table, opening wine, putting out place settings and making sure Heather and Jim's son (the strawberry eater) brushes his teeth before going to bed. Noemi, a park ranger who is pregnant with Jim's second child, offers me some bread and cheese.

The group's network of relationships is fairly typical in polyamorous circles, where it's not unusual to hear somebody introduce a "husband's girlfriend" or "my wife and her boyfriends". Noemi does her best to explain the history of the family, but it sounds like a logic puzzle. "If you really want to understand all of our relationships, it might be easier if we drew you a chart," says Heather (see Diagram). "I'm not dating any of them," says Alicia, a librarian. "My boyfriend is poly, so I guess I'm poly by association."

"I feel like I'm monogamous because I've been sleeping with only one person for about five years," says Noemi. Everybody starts laughing, and finally she admits, "OK, well I did sleep with some other people too."

It is hard to estimate how many polyamorists exist – there is no box for them on any national census – but the number of online resources, articles and books on the topic has exploded since the early 1990s, when the term polyamory ("poly" for short) was coined in internet newsgroups. The Ethical Slut, a 1997 book by Dossie Easton and Catherine Liszt that some call the "bible of poly", has sold more than 50,000 copies and is about to go into its second edition. Recently the concept of multiple lovers has become the subject of public debate in the US, where conflicts over gay marriage have led some conservatives to claim that homosexual weddings will lead to marriages of more than two people: if you can have two mothers, they say, why not two mothers and a father?

For psychologists and evolutionary biologists, polyamory is a rare opportunity to see, out in the open, what happens when people stop suppressing their desire for multiple partners and embrace non-monogamy. Proponents say the poly brand of open but committed relationships may be a way around infidelity because it turns an age-old problem into a solution: polyamorists are released from the burdens of traditional marriage vows, yet they seem to keep their long-term relationships intact. What makes poly enticing is the possibility of reconciling long-term stability and romantic variety.No swinging, please

And why shouldn't we consider it? When most people think of non-exclusive marriages, they think of polygamy, an ancient but still widespread practice that involves one person, usually male, acquiring multiple spouses in a harem-like arrangement. Or swinging, in which couples have casual flings on the side. Polyamory is different. It encompasses a dizzying variety of arrangements – anything from couples with long-term lovers on the side to larger groups with overlapping relationships. If anything characterises poly, says Elaine Cook, a psychiatrist who has a private practice in Marin county, California, it is a lack of rigid structure.

What evidence there is shows that poly couples stay together as long as monogamous ones – and, apparently, for good reasons. In a study published last December in the Electronic Journal of Human Sexuality (vol 8), Cook analysed the relationships of seven couples who had been married for more than 10 years, and who had had additional partners for at least seven of those years. She found that most of the couples reported "love" or "connection" as important reasons for staying together. This contrasts with monogamous couples, Cook notes, who often list external factors such as religion or family as major reasons for remaining committed.

That is telling. Cook speculates that polyamorists perceive themselves as having more choices, and therefore they only stay in marriages and relationships that make them happy. "They have other relationships that they are perhaps equally excited about being in, but they want to maintain this [marriage] relationship because it continues to satisfy them," she says.

For some, poly may be more realistic than monogamy. Having multiple partners frees people from the process of trying to find "the one" who is perfect for them in every way. In April, psychologist Rachel Robbins at the Mission Mental Health clinic in San Francisco conducted a survey of 250 polyamorous women. The number 1 reason they gave for being poly was "to experience different activities and explore different parts of themselves with different people". Instead of asking one person to meet all their needs, polyamorists are content with several people who each meet a few.

Noemi's housemates would drink to that. "I have a lot of interests and passions in my life, and I can't fulfil them all in my relationship," says Alicia. "It was good to have my partner go off and date other people, because then I could pursue my outside interests too – and I didn't feel scrutinised for wanting to do that." Noemi agrees: "It makes me sad that so many people isolate themselves," she says. "It's good to have multiple people who love you, and it's good to have freedom and downtime too."

All well and good, but what about the demands of juggling so many commitments at once? Surely it saps their time and energy. In a break during dinner, I ask how the family manages multiple relationships, particularly as most of them live under the same roof.

"We all have our own bedrooms, which is key," Noemi says. "And our bedrooms aren't next to each other, so we have privacy," says Heather. "Also, we have a nominal schedule where Jim sleeps with Noemi and me on an every-other-night basis, and I'm with Gordon on the weekends."

"My nights without Jim are great," Noemi says with a laugh. "I get to hog the covers, and nobody snores."

Critics call poly self-indulgent and morally reprehensible. Yet it is hardly a sexual free-for-all. The freedom has limits – and managing emotions like jealousy becomes a central issue. "These are designer relationships," Cook says. "Every group decides for itself what's open and what isn't."

Take Emma and Nate, a young married couple living in California's Silicon Valley who describe themselves as "stable and well-settled". They met in college 11 years ago and have always had a polyamorous relationship. Emma has had a boyfriend for the past seven years, while Nate prefers to have short-term romances with friends. Some aspects of their relationship, however, are not open. "We don't do sleepovers with other people," Emma says.

"I like waking up next to her in the morning," Nate says. "The only exception is if I'm out of town, in which case I don't mind if she's having a sleepover." Another rule they have established is letting each other know in advance about dates with other people. "If either of us gets serious about someone else, we bring them home to meet the spouse," says Nate. "In fact, that's what we're doing tomorrow – we're having lunch with my new girlfriend and her husband."Your cheating heart

Polyamorists come to it at different points in their lives and for different reasons. Emma says she had open relationships in high school, and many people I spoke with described discovering poly in their late teens or early twenties. Most, like Jim, tried monogamy. "My first marriage was supposed to be monogamous, and I was," he recalls. "But she slept around in a cheating way. That killed the relationship."

So is poly more sustainable than monogamy? "Infidelity in monogamous relationships is estimated at 60 to 70 per cent, so it seems that attraction to more than one person is normal. The question is how we deal with that," says Meg Barker, a professor of psychology at London South Bank University who presented her research into poly at the 2005 meeting of The British Psychological Society. "The evidence is overwhelming that monogamy isn't natural," says evolutionary biologist David Barash of the University of Washington, Seattle. "Lots of people believe that once they find 'the one', they'll never want anyone else. Then they're blindsided by their own inclinations to desire other attractive individuals. So it's useful to know that this behaviour is natural."

But as a mating strategy, poly may not be any better than monogamy; a person's reproductive success may diminish if there is less pressure to be exclusive. "Jealousy is probably fitness enhancing," Barash says. A more jealous male is likely to stick closer to his mate and prevent her from getting impregnated by other males. "A good look at human biology does not support polyamory any more than it supports monogamy," he says. Biologist Joan Roughgarden, at Stanford University in Palo Alto, California, goes further. "Polyamory won't last. The likelihood of being able to successfully raise children in that context is very limited. My guess is that it's not an evolutionary advance, but a liability."

To others, however, biology is not the point. "In middle-class urban cultures, people aren't marrying for survival any more," says psychologist Dossie Easton, co-author of The Ethical Slut. "They can get divorced, and the kids won't starve. This means we're having marriages and relationships for very different reasons than our ancestors did. We're doing it for emotional gratification." Easton sees poly as a break from the "survival strategy" traditions that created both polygamy and monogamy. "Polyamory is a cultural outgrowth of serial monogamy, or having multiple partners without necessity," she says. "Once you're released from necessity, you can start doing all kinds of original thinking."

Barker concurs. "It's assumed that jealousy is a natural response," she says, "but some polyamorous people say they hardly feel it at all. I think this gives us insight into how people can make sense of their worlds in many ways if monogamy isn't the default." She has found that when people leave traditional monogamy behind, they often rethink "givens" such as how to divide up the housework, money and childcare. Children of poly couples, for instance, tend to be raised by a small community instead of two parents.

Back in San Francisco, Heather's family is clearing the table. As she replaces our plates with bowls of fruit compote, she says poly is a way of keeping her long-term partnerships alive. "When you think about it, what happened is that Jim and I didn't get divorced when we got new partners. We're still together and yet have more love from other people."

"Polyamory is not for everybody," says Jim. "But it creates a range of options, which is important because you can't optimise one kind of relationship to fit everyone."

"The important thing is that we trust each other," says Noemi, rubbing her pregnant belly with a smile. Although poly is still well out of the mainstream, it has become an attractive alternative to monogamy for some. Whether it is good for society remains an open question. For now, there's a more pressing issue – is it good for you?